1. Infrared Spectroscopy
L.O. To understand the principles of IR spectroscopy.
To be able to interpret simple IR spectra.

2. Infrared Spectroscopy
Different covalent bonds have different strengths as a result of the masses of the different atoms at either end of the bond.
As a result, the bonds vibrate at different frequencies
The frequency of vibration can be found by detecting when the molecules absorb electro-magnetic radiation.
The different types of vibrations include... STRETCHING and BENDING
SYMMETRIC BENDING ASYMMETRIC
STRETCHING STRETCH

3. The spectrophotometer
Process:
1. A beam of infra red radiation is passed through the sample and a similar beam is passed through the reference cell
2. The frequency of radiation is varied and bonds vibrating with a similar frequency absorb the radiation
3. The amount of radiation absorbed by the sample is compared with the reference and the results are collected, stored and plotted

4. The Infrared Spectrophotometer
A bond will absorb radiation of a frequency similar to its vibration(s)
normal vibration
vibration having absorbed energy

5. USES
Particular bonds can be identified if they have characteristic peaks in specific, identifiable parts of the spectrum. For example, O-H and C=O.
Compounds can be identified by comparing spectra. The only way to do this using IR alone is to compare its spectrum with a known reference sample. The “fingerprint region” of the IR spectrum is unique to each compound.
Infra-red spectra are complex as each molecule can have many different vibrations occurring.
Total characterisation based only on a substances’ IR spectrum is almost impossible unless you have computerised data handling facilities for comparison of the obtained spectrum with known ones in the computer memory.
The technique is useful when used with other methods -nuclear magnetic resonance spectroscopy and mass spectroscopy.

6. Interpretation
The position of peaks depends on bond strength and the masses of the atoms joined by the bond.
For example:
strong bonds absorb at lower wavenumbers
Light atoms absorb at lower wavenumbers
weak bonds absorb at high wavenumbers
heavy atoms absorb at high wavenumbers

7. Interpretation
Vertical axis Absorbance the stronger the absorbance the larger the peak
Horizontal axis Frequency wavenumber (waves per centimetre) / cm-1
Wavelength microns (m); 1 micron = 1000 nanometres

8. Fingerprint region
• organic molecules have a lot of C-C and C-H bonds within their structure
• spectra obtained will have peaks in the 1400 cm-1 to 800 cm-1 range
• this is referred to as the “fingerprint” region
• the pattern obtained is characteristic of a particular compound the frequency
of any absorption is also affected by adjoining atoms or groups.

9. IR spectrum of a carbonyl compound
N.B:
Carbonyl compounds show a sharp, strong absorption between 1700 and 1760 cm-1, which is due to the presence of the C=O bond

10. IR spectrum of an alcohol
N.B:
Alcohols show a broad absorption between 3200 and 3600 cm-1, which is due to the presence of the O-H bond

11. IR spectrum of a carboxylic acid
N.B:
Carboxylic acids show a broad absorption between 3200 and 3600 cm-1, which is due to the presence of the O-H bond.
They also show a strong absorption around 1700 cm-1, which is due to the presence of the C=O bond

12. IR spectrum of an ester N.B:
Esters show a strong absorption between 1750 cm-1 and 1730 cm-1, which is due to the presence of the C=O bond

13. What is it? ALCOHOL ALDEHYDE CARBOXYLIC ACID
One can tell the difference between alcohols, aldehydes and carboxylic acids by comparison of their spectra.
ALCOHOL
O-H STRETCH
ALDEHYDE
C=O STRETCH
O-H STRETCH
C=O STRETCH
AND
CARBOXYLIC ACID

14. Characteristic frequencies
N-H
CN
C-Cl
O-H
C=O
C-O
C-H
Aromatic C-C
C=C
C-C alkanes

15. Characteristic absorption frequencies
Bond Class of compound Range / cm-1 Intensity
C-H Alkane strong
C-C Alkane weak
C=C Alkene variable
C=O Ketone strong
Aldehyde strong
Carboxylic acid strong
Ester strong
Amide strong
C-O Alcohol, ester, acid, ether strong
O-H Alcohol (monomer) variable, sharp
Alcohol (H-bonded) strong, broad
Carboxylic acid (H-bonded) variable, broad
N-H Amine, Amide (approx) medium
CN Nitrile medium
C-X Chloride strong
Bromide strong
Iodide (approx) strong